GEOLOGICA BELGICA (2008) 11: 3-24

Early and MiddlE OrdOvician acritarchs Of thE sEnnE-sEnnEttE rivEr vallEys (BraBant Massif, BElgiuM)

and thEir stratigraphic iMplicatiOns

Michel VANGUESTAINE

(6 figures, 1 table, 2 plates)

Paléobotanique, Paléopalynologie et Micropaléontologie, Université de Liège, Belgique.E-mail: M.Vanguestaine@ulg.ac.be

aBstract. Early and Middle Ordovician strata from the Senne-Sennette river valleys, at the southern border of the Brabant Massif, have been intensively sampled for an acritarch study. Previous research, together with unpublished and new observations, are reported. Three sections are studied in the Sennette valley: along the Bruxelles-Charleroi canal south of Asquempont, in the Virginal railway trench and at La Tourette along the bief 29 of the old canal. Two other locations are also considered: a section along the Coeurq river and another along the railway and in the quarry at Quenast, in the Senne valley. An early Tremadocian assemblage is observed in the three localities of the Sennette valley within strata which biostratigraphical content allows their lithostratigraphical assignment to the Chevlipont Formation. Between the latter formation and the Rigenée Formation, rocks coeval to the Abbaye de Villers and Tribotte Formations of the Thyle valley, yielded two distinct assemblages. The Frankea hamata-Striatotheca rarirrugulata Acritarch Zone of the English Lake District, indicating a late Arenig age, is observed in sediments reported to the Abbaye de Villers Formation from the Virginal railway section. The lithostratigraphical assignment is confirmed as the same Zone is recorded from coeval strata from the Thyle section. A second assemblage, characterized by the co-occurrence of species such as Arbusculidium filamentosum and Frankea sartbernardensis, is recognized in an unnamed member attributed to the Abbaye de Villers Formation at Asquempont and in strata attributed to the Abbaye de Villers Formation in the Quenast railway section. In these two localities of Asquempont and Quesnast, chitinozoans indicating respectively a middle and a late Arenig age are also described. According to acritarchs, the age of those strata would be late Arenig. Combined acritarch and chitinozoan results indicate a probable late Arenig age. Moreover, due to their acritarch content, the lithostratigraphical attribution of the Asquempont and Quenast strata is questioned. They are coeval with the Tribotte Formation or with a level at the boundary between the Tribotte and the Rigenée Formations of the Thyle river section. Other ‘pre-Rigenée’ localities could also belong to the A. filamentosum-F. sartbernardensis assemblage, based on the presence of F. sartbernardensis or A. filamentosum. One locality is situated at Asquempont and consists of the main part of the section studied by Martin & Rickards (1979) and another is located at La Tourette. Samples of the Rigenée Formation have not revealed the stratigraphical markers known in the type section of the Thyle valley despite the great number that were investigated in different localities from the Senne-Sennette valleys.

KEyWOrds: Acritarch biostratigraphy, chitinozoan biostratigraphy, Abbaye de Villers Formation, Chevlipont Formation.

1. introduction

Previous acritarch studies in Lower and Middle Ordovician strata of the Thyle river section (Martin, 1977; André et al., 1991; Servais, 1991; Vanguestaine & Léonard, 2005) allowed the recognition of three successive acritarch assemblages: 1. an early Tremadocian assemblage, in the Chevlipont Formation, characterized by the abundance of diacromorph species amongst which Acanthodiacrodium angustum, of galeate species assigned to the genera Cymatiogalea and Stelliferidium and of vulcanisphaerid species as Vulcanisphaera flagellum; 2. the late Arenig Frankea hamata-Striatotheca rarirrugulata Acritarch Zone in the middle and upper part of the Abbaye de Villers Formation and the lower part of the Tribotte Formation; 3. the successive appearance of Frankea sartbernardensis, Striatotheca quieta, Arkonia virgata and Frankea

hamulata from the upper part of the Tribotte Formation up to the middle part of the Rigenée Formation. The Arenig-Llanvirn boundary is probably to be found within this last sequence.

Coeval strata also exist in the Senne-Sennette river valleys, 30 km west of the Thyle valley. They are herein studied in three sections from the Sennette valley (the Bruxelles-Charleroi canal section, the Virginal railway section and the La Tourette section), in the Coeurq river valley and in the Senne valley (Quenast railway section and Quenast quarry). Their rocks have been assigned to the Lower and Middle Ordovician formations defined in the Thyle valley (Verniers et al., 2002). From base to top, the succession is composed of the Chevlipont Formation (early Tremadocian), the Abbaye de Villers Formation (late Arenig), the Tribotte Formation (late Arenig-early Llanvirn) and the Rigenée Formation (late

4� Michel Vanguestaine

Arenig-Llanvirn). The ages are according to Vanguestaine (this volume). In the studied area, the Asquempont fault forms the contact between Ordovician rocks and the Lower-Middle Cambrian Oisquercq Formation. Due to the tectonic complexity of the area (Debacker et al., 2003), the stratigraphical order, the thickness estimation and the lithostratigraphical assignment of the Lower-Middle Ordovician rocks is problematical. Local names and informal units have therefore been used in the literature.

Leriche (1912) used the term “quartzophyllades de Virginal” and assigned them to the early Tremadocian, which revealed correct much later (Vanguestaine, 1978), for finely laminar siltstones (now the Chevlipont Formation) in the Virginal railway section.

Legrand (1967) described the Lower-Middle Ordovician rocks along the Bruxelles-Charleroi canal south of Asquempont. From base to summit, he distinguished four lithological units: 1. black siliceous shales; 2. grey-green sandstones with trace of fossils; 3. laminar siltstones; 4. blackish shales with graptolites and inarticulated brachiopods. Units 1, 2 and 3 were considered as Arenig (now assigned to the Abbaye de Villers and Tribotte Formations); unit 4 as Llanvirn (now the Rigenée Formation). For the same succession along the Bruxelles-Charleroi canal south of Asquempont, Martin & Rickards (1979) used informal units (A-D) between km 40.079 and km 39.776, the last locality containing a graptolite level.

Beugnies in Waterlot et al. (1973) considered a black unit (now ‘unnamed member’ of the Abbaye de Villers Formation in Verniers et al, 2002), immediately south to the Asquempont fault, as belonging to the Revin Group and used the term Rv2, infering a Cambrian age. This unit is actually Middle Ordovician in age (Vanguestaine, 1978 and Samuelsson & Verniers, 2000). Its precise age will be discussed later (Section 4.3.). Moreover Beugnies in Waterlot et al. (1973) introduced a Quenast unit (‘Quartzophyllades zonaires de Quenast’) for laminar siltstones and sandstones in the railway section at Quenast. Lenoir (in Lenoir et al., 1989 and in André et al., 1991, fig. 16) also used this term and extended it to encompass all units comprised between the Chevlipont Formation (= Virginal Formation in André et al., 1991) and the Rigenée Formation (= La Tourette Formation in André et al., 1991) in the studied area.

In the Virginal railway section, Vanguestaine (1978) reported the existence of two acritarch assemblages: an early Tremadocian in the ‘quartzophyllades de Virginal’ (now the Chevlipont Formation) and a Middle Ordovician one (dated as Arenig-Llanvirn at the time ) in the overlying strata (now the Abbaye de Villers Formation). He also recorded an Arenig-Llanvirn assemblage immediately south to the Asquempont fault in the canal section (in strata reported now to the unnamed member of the Abbaye de Villers Formation, Verniers et al., 2002) and in the Quenast railway section. Middle Ordovician rocks of the Virginal railway section will be demonstrated to be older than the Middle Ordovician rocks of Asquempont and part at least of the rocks of Quenast (Sections 4.2. & 4.3.).

Martin & Rickards (1979), in their study of the Ordovician and Silurian rocks of the Bruxelles-Charleroi canal section, also examined some of the Middle Ordovician strata of concern in the present paper. They used, as already said, informal units (A, B, C, D, E, F) for the studied Ordovician rocks. Because units E and F are of late Ordovician age (Martin & Rickards, 1979, Verniers et al., 2002), only the units A to D will be considered herein. However few acritarch species are identified by Martin & Rickards (1979) and taken as indicative of a broad ?late Arenig-Llanvirn age. This age is locally confirmed by the restudy of Didymograptus artus graptolite level in the unit D (Maletz & Servais, 1998). Stratigraphically important taxa as Arbusculidium filamentosum, Frankea sartbernardensis, Vogtlandia multiradialis were however determined and their meaning will be discussed (Section 4.3.).

Lenoir (1987 and in André et al., 1991) examined new samples from several sections of the Senne-Sennette valleys. He completed the observations of Vanguestaine (1978) in finding the lower Tremadocian acritarch assemblage in two other localities (Asquempont and La Tourette) and the Middle Ordovician acritarch assemblage at La Tourette, in the Coeurq section and in the Quenast quarry. His unpublished work is incorporated in the present paper and modified according to the new stratigraphic progress made in the Thyle valley Vanguestaine (this volume) and the observations of new samples.

Samuelsson & Verniers (2000) described chitinozoans in three of the studied units: a mid Arenig assemblage in the unnamed member of the Abbaye de Villers Formation at Asquempont; a late Arenig assemblage in the Abbaye de Villers Formation in the Quenast railway section; a late Arenig-early Llanvirn assemblage in the Rigenée Formation from the canal section south of Asquempont.

Hennebert & Eggermont (2001) and Debacker et al. (2003) mapped the eastern part of the studied area (Sennette valley) and provided useful cross-sections for the present work.

The aim of the present paper is to review the different sections studied so far and their acritarch content in the Senne-Sennette valleys and to establish comparisons with coeval strata in the Thyle section and with the chitinozoan results.

2. section locations, studied samples and palynological techniques

Five sections have been sampled, three in the Sennette valley, one in the Coeurq and the fifth one in the Senne valley (Fig. 1).

2.1. Sections in the Sennette valley

The three studied sections are the canal section south of Asquempont, the Virginal railway section and the La Tourette section along the bief 29 of the old canal.

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 5

2.1.1. The Bruxelles-Charleroi canal section south of Asquempont

The digging of the new Bruxelles-Charleroi canal has created important outcrops amongst which the one south of Asquempont. On the eastern side of the canal, the fault contact between Lower and Middle Ordovician rocks and Lower to lower Middle Cambrian rocks of the Oisquercq Formation is observable (Legrand, 1967; Hennebert & Eggermont, 2002; Debacker et al., 2003).

Several faults rendered the local structures very complicated (Debacker, 2001; Hennebert & Eggermont, 2002). The Asquempont fault has been thoroughly studied and redefined by Debacker et al. (2003) who distinguished pre-cleavage (the redefined Asquempont fault) and post-cleavage faults (amongst which the Asquempont fault sensu Legrand, 1967) associated to the Nieuwpoort-Asquempont fault zone (De Vos et al., 1993). Legrand (1967) reported the discovery of a graptolite level in a shaly unit, now assigned to the Rigenée Formation and

identified the lower Llanvirn Didymograptus artus graptolite Zone, confirmed by Martin & Rickards (1979) and Maletz & Servais (1998). Except for a few meters of Tremadocian rocks crushed by the Asquempont fault found by Lenoir (1987), Legrand (1967) postulated the existence of a complete Middle Ordovician succession comprising from base to top four units: 1. black siliceous shales; 2. grey-green sandstones with traces of fossils; 3. laminar siltstones; 4. blackish shales (see Table 1).

The unnamed member of the Abbaye de Villers Formation of Verniers et al., (2002) corresponds to the lower part of the unit 1 of Legrand (1967). Units A, B, C and D of Martin & Rickards (1979) corresponds to the units 1 to 4 (except the unnamed member) of Legrand (1967).

The succession, very briefly described by Beugnies in Wartelot et al. (1973) at Asquempont, comprised the unnamed member taken as a unit belonging to the Revin Group (Rv2). The overlying units either have

figure 1: Sample location in the Lower and Middle Ordovician strata of the Coeurq, Senne-Sennette river valleys. A = Asquempont, C = Coeurq, Q = Quenast, T = La Tourette, V = Virginal. The Asquempont fault (sensu Debacker et al., 2003) is according to the mapping of Lenoir (1987) and indicated by a double line.

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never been observed by later workers (his Sm 1, green, shale formation) or are not clearly characterized (his Sm 2 = the Quenast formation, and Sl1a, a formation described as composed of grey green shales and sandy slates). However his Llanvirn Sl1b unit is recognized later and corresponds to the Rigenée Formation.

Hennebert & Eggermont (2002) assigned to the Abbaye de Villers Formation, most of the rocks comprised between the Asquempont fault to the north and the Rigenée Formation to the south, except some faulted Tribotte strata at mid distance. Mapping of Debacker et al. (2003) in the Asquempont area did not distinguished the Abbaye de Villers from the Tribotte Formations and they are grouped in one unit.

It will be demonstrated (Section 5.1.) that most of the rocks comprised between the Chevlipont and the Rigenée Formation are coeval with the Tribotte Formation or with strata at the boundary between the Tribotte and Rigenée Formations.

Thirteen samples are taken in this section (location in Appendix 1).

2.1.2. The Virginal railway sectionThe section, localized about 1 km west of the Asquempont section, has been recently restudied in detail by Debacker et al. (2003). From north to south, the section exhibits the Asquempont Member of the Oisquercq Formation, at km

8.185 of the old railway the Asquempont fault, between km 8.185 and 8.235 the Chevlipont Formation, from km 8.235 to 8.380 the Abbaye de Villers Formation and between km 8.380 and 8.575 the Rigenée Formation. Ten samples have been studied (location in Appendix 1).

2.1.3. The La Tourette sectionThe section lies at the northern side of an arm of the old canal, called “bief 29”, in the garden of the house with number 11. It is accessible by the road along the new canal. From east to west, it exhibits several small outcrops belonging to the Oisquercq, Chevlipont and the unnamed member of the Abbaye de Villers Formation (Lenoir, 1987). A slaty breccia between the outcrops of the Oisquercq and the Chevlipont Formation is a striking feature probably corresponding to the Asquempont fault. A galena dyke and a quartz vein are also observable in an outcrop within the property of the house with number 13. Eleven samples were taken (location in Appendix 1).

2.2. The Coeurq river valley section

Dispersed outcrops of laminar siltstones and slates are observable along the Coeurq river just north of the Coeurq village. They have been mapped by Beugnies in Waterlot et al. (1973) as belonging to the “Quenast quartzophyllades” and by Lenoir (1987) in his Quenast formation but will be referred in the present work to an undefined formation. An

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 7

outcrop of the Rigenée Formation is visible also at the “Ferme des Aunois” at 600 m northeast of the Hennuyères station. Seven samples are taken (location in Appendix 1).

2.3. The Quenast railway section and quarry

Discontinuous outcrops are located north of the village of Quenast village, along the eastern side of the railway trench between km 3.3 and 4.0. From north to south, the section exposes the Oisquercq Formation, a transitional zone over some 7 m (Vanguestaine, 1978) and from km 3.384 to 4.0, near the Quenast station, folded grey laminar siltstones and sandstones. The Asquempont fault would be located at km 3.366 where dark grey Ordovician rocks are in contact with pale grey Cambrian rocks (Debacker et al., 2003). The laminar siltstones are the “Quartzophyllades zonaires de Quenast” of Beugnies in Waterlot et al. (1973). They were placed in the Abbaye de Villers Formation by Samuelsson & Verniers (2000) and by Hennebert & Eggermont (2002). It will be demonstrated that in one locality at least (Quenast – 3.770) the acritarch assemblage does not match the acritarch composition known in the Abbaye de Villers type locality. The rocks are herein maintained in the “Quartzophyllades zonaires de Quenast” as defined by Beugnies in Waterlot et al. (1973). In the latter paper, they were lithologically compared to the strata yielding the Tremadocian graptolites in the Thyle valley (the Tangissart member of the Mousty Formation and/or the Chevlipont Formation). Vanguestaine (1978) found however an Arenig-Llanvirn age and Samuelsson & Verniers (2000) a late Arenig age.

Samples were also taken in the Quenast quarry in strata supposedly belonging to the Rigenée Formation (Beugnies in Waterlot et al., 1973; Lenoir, 1987). The samples are at the contact with a magmatic body interpreted as a pipe of microdioritic composition. The intrusion has a more or less elliptical boundary of 2.0 km by 1.4 km (André et al., 1991).

Outcrops of a unit intercalated between the “Quartzophyllades zonaires de Quenast” and the Rigenée Formation, described by Beugnies in Waterlot et al. (1973) as composed of grey black shales and sandy slates have not been sampled for the present project.

In total 15 samples have been collected in the Quenast section (see location in Appendix 1).

2.4. Methods

Techniques used to isolate the organic matter are described in Streel (1965) and correspond to the standard methodology in the Palynological Laboratory of Liège University. The slides are housed in the Collections of the

Laboratory of Palynology, Liège University.

3. palaeontological results

In total, 60 samples were processed for acritarch studies. Forty-six yielded microfossils, mainly acritarchs but also chitinozoans and scolecodont, the latter two are not further considered. The organic matter is generally very poorly preserved and most acritarch specimens are underterminable. About four hundred acritarch specimens, all samples together, were however determined to a specific level, although sometimes in open nomenclature.

figure 2 : Acritarch species distribution in the Lower and Middle Ordovician of the Asquempont canal section. Underlined samples designate samples already studied in Vanguestaine (1978). Note that in the latter paper sample Asquempont – 5, yielding Middle Ordovician acritarch species, is not reported herein. Units A and B are from Martin and Rickards (1979).

8� Michel Vanguestaine

Their identification and stratigraphical distribution in each section is presented in Figures 2 to 6. Although many other species are present, only a few selected taxa are reported in the figures. Studies of Martin (1977), Servais (1991) and Vanguestaine & Léonard (2005) in the Thyle section describe and illustrate most of the species discussed herein. Two plates illustrate the main taxa encountered in the Senne-Sennette valley. The identification of Vanguestaine (1978) have generally been controlled by us except for two samples of the Quenast railway section in which some key species as Frankea sartbernardensis could not be found again. Their identification in those samples remains therefore insure.

3.1. Acritarch species distribution in the studied sections

3.1.1. Asquempont section (Fig. 2)Palynological results demonstrate a clear distinction between samples Asq-3 and Asq-5 on one hand and the other samples on the other hand. The transition from one assemblage to the other is observed between sample Asq-5 to Asq-5,5, i.e. within half a meter. This observation matches very well with the profile provided in Debacker et al. (2003) where the boundary between the Chevlipont Formation and the unnamed member is located about 5 m south of our point O, the quartz veins (see Appendix 1). The best diversified samples (Asq-7 and Asq-11) are in the unnamed member. They are much better than samples higher in the sequence in units A, B and in the Rigenée Formation which yielded very few identifiable specimens as also mentioned in Martin & Rickards (1979).

3.1.2. Virginal railway section (Fig. 3)Once again the difference in composition, already emphasized in Vanguestaine (1978), between the lower samples from the Chevlipont Formation and from overlying strata is particularly clear. Sample 8.343, newly studied, has yielded a very diversified assemblage.

3.1.3. La Tourette section (Fig. 4)Here again, the same clear distinction into two assemblages is evident. The transition from one assemblage (sample –6) to the completely different assemblage (sample –7) was observed within a few decimeters lacking any notable tectonic disturbance. However, it should be noted that one acritarch species, Acanthodiacrodium angustum (Pl. I, Fig. 3), commonly found in the lower assemblage (La Tourette –1 to – 6) is also found as a single specimen in La Tourette – 7 (Pl. I, Fig. 2) where it is mixed to species of the second assemblage. Debacker et al. (2003) did not mention any fault in this section at the contact between their mapped units Chevlipont and Abbaye de Villers – Tribotte.

3.1.4. Coeurq river section (Fig. 5)Very few results were obtained from samples of this section. They indicate however a post-early Tremadocian age and therefore a correlation of the undefined formation with the Abbaye de Villers or Tribotte Formations of the Thyle river valley.

3.1.5. Quenast railway and quarry section (Fig. 6)The so-called Transitional Zone is nearly completely

figure 3: Acritarch species distribution in the Lower and Middle Ordovician of the Virginal railway section.Underlined samples are from Vanguestaine (1978).Virginal –8.343 is a newly studied sample.

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 9

figure 4: Acritarch species distribution in the Lower and Middle Ordovician from La Tourette section.

figure 5: Acritarch species distribution in the Lower and Middle Ordovician from the Coeurq river section.

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barren of acritarchs or yielding specimens without any clear stratigraphic meaning. No clear distinction is observable between the “Quenast quartzophyllades” and the overlying Rigenée Formation. Sample –2B located at 10 m from the contact with the microdioritic pipe, yields remarkably preserved organic matter despite the proximity of the magmatic intrusive body.

3.2. Systematic Palaeontology

Genus Acanthodiacrodium Timofeev, 1958 emend. Deflandre & Deflandre – Rigaud, 1962

Acanthodiacrodium tasselii Martin, 1969Pl. I, Figs 13 & 14

1969 Acanthodiacrodium tasselii n. sp. in Martin, p. 126, pl. V, figs 223, 229 & 231; pl. VI, fig. 283?1977 Acanthodiacrodium tasselii Martin, 1969, in Martin p. 21, pl. IV, fig. 25; text-fig. 11

Remarks:1. The best recorded specimen (Pl. I, Fig. 13) has a

vesicle length of 25 µm. It bears about 20-25 processes at each pole. The maximum length of the processes is about 14 µm. The processes are in form of narrow cylindrical filaments.

2. The species is reported by Martin (1969) as occurring at Rigenée in the lowermost part of the Rigenée Formation. Vanguestaine (this volume) reports the same occurrence in coeval strata at Rigenée but does not find the species in the Abbaye de Villers Formation unlike Martin (1977).

Occurrence: Unnamed member of the Asquempont canal section; Abbaye de Villers Formation of the Virginal railway section; questionable specimens in the undefined formation of the Coeurcq river section.

Genus Adorfia Burmann, 1970Adorfia firma, Burmann, 1970

Pl. I, Fig 15, Pl. II, Fig. 1

1970 Adorfia firma n. sp. in Burmann, p. 295, pl. 5, figs 3-4

Remark:The figured specimens are left in open nomenclature because they do not show the pedunculate balls in the ultimate pinnulae.

Occurrence: The species is found in the “Quartzophyllades de Quenast” (Quenast – 3.770) and identified with doubt in two others (Virginal – 8.349 and Asquempont – 7).

Genus Arbusculidium Vavrdová, 1972Arbusculidium filamentosum (Vavrdová, 1965)

Vavrdová, 1972Pl. II, Fig. 2

1965 Dasydiacrodium filamentosum n. sp. in Vavdrova, p. 335-336; pl. 4, fig.11972 Arbusculidium filamentosum (Vavrdová, 1965) nov. comb. in Vavrdová, p. 81

Remark:Vanguestaine (this volume) discussed the distribution of

figure 6: Acritarch species distribution in the Lower and Middle Ordovician from the Quenast railway and quarry section. Underlined samples are from Vanguestaine (1978). Sample Quenast 3.770 is a newly studied sample. Vanguestaine (1978) determination of Frankea sartbernardensis in the samples Quenast –3.386 and –3.650 could not be confirmed (white circle). Exact location of sample –3.650 is discussed in Appendix 1.

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 11

the species in the Asquempont canal section according Martin & Rickards (1979) and its stratigraphic meaning with respect to the Arenig-Llanvirn boundary.

Occurrence: Unnamed member in the Asquempont canal section (Asquempont – 7); quartzophyllades zonaire de Quenast (Quenast – 3.700); ?Abbaye de Villers Formation at Virginal (Virginal – 8.343).

Genus Arkonia Burmann, 1970Arkonia virgata Burmann, 1970

Pl. II Fig. 3

1970 Arkonia virgata n. sp. in Burmann, p. 298, pl. 7, fig. 6non 1977 Arkonia virgata Burmann, 1970 in Martin, p. 121991 Arkonia virgata Burmann, 1970 in Servais, p. 240-241, pl. 1, fig. 221991 Arkonia virgata Burmann, 1970 in André et al., p. 2901992 Arkonia virgata Burmann, 1970 in Servais & Maletz, p. 270, pl. 3, fig 121993 Arkonia virgata Burmann, 1970 in Maletz & Servais, p. 140, 142, fig 4:18

Remarks:1. The distinction between two of the commonest species

of the genus, Arkonia tenuata and Arkonia virgata lies in the number and the thickness of the ribs orning the external surface of the central triangular body. According to Burmann (1970), A. tenuata is covered with fine ribs, 10-12 in number (including the calked ribs) parallel to the adjacent sides. A. virgata bears relatively thick ribs, about 7 in number (including the calked ribs) parallel to the adjacent sides.

2. The specimen figured Pl. II, Fig. 3 is clearly an Arkonia virgata as the ribs are widely spaced. Specimen of Pl. II, Fig. 4 seems more densely striated and would be better placed in a transitional position between virgata and tenuata.

3. Martin (1977, p.12) cited the species in the Abbaye de Villers Formation of the Brabant Massif. No illustration was however provided. Its identification is problematic because it was not observed afterwards (Vanguestaine, this volume).

4. According to Servais (1997), the species is unknown in proven Arenig rocks.

Occurrence: Unnamed member of the Asquempont canal section (Asquempont – 7).

Genus Coryphidium Vavrdová, 1972Coryphidium spp.

Pl. II, Fig. 7

Remark: Due to the poor state of preservation, distinction at the specific level is very difficult and is generally not achieved, except in sample Virginal –8.343 where

Coryphydium aff. bohemicum sensu Molyneux and Leader (1997) is clearly recognized. See Vanguestaine (this volume) for identification of the latter species.

Occurrence: Ubiquitous.

Genus Dasydorus Playford & Martin, 1984Dasydorus sp. cf. Dasydorus cirritus

Playford & Martin, 1984Pl. II, Fig. 8

1984 Dasydorus cirritus sp. nov. in Playford & Martin p. 198, fig. 6A-C1988 Dasydorus cirritus Playford & Martin in Arriagha e Cunha & Vanguestaine, p. 72-73; pl. I, figs 12, 13; pl. II, figs 1-4

Description: elliptical vesicles with a granular like ornamentation of Lophosphaeridium aspect except that a smooth polar zone can be evidented on some specimens. Ornaments of ½ - ¾ µm in diameter.

Remarks:1. D. cirritus bears baculum-like projections, 1-3.8 µm

long. Whether or not the herein observed specimens have shorter ornaments than D. cirritus or that they are truncated projections is unknown and justifies their description in open nomenclature.

2. The species is described in the Goldwyer Formation of Western Australia of poorly constrained age: late Arenig - ?early Caradoc (Playford & Martin, 1988). It is also well illustrated in the “Xistas com Phyllodocites” Formation, Southeast Portugal dated as late Arenig (Didymograptus hirundo) by graptolite. (Arriagha e Cunha & Vanguestaine, 1988). The acritarch content (?Arkonia virgata, Striatotheca quieta, Stellechinatum cf. celestum) could however be in favour of an early Llanvirn age.

Occurrence: Unnamed member in the Latourette section.

Genus Frankea Burmann, 1970Frankea sartbernardensis (Martin, 1966) Colbath, 1986

Pl. II, Figs 9 and 10

non 1977 Frankea sartbernardensis (Martin, 1965) Burmann, 1970, in Martin, pl. 5, fig. 71978 Frankea sartbernardensis Martin, 1966 in Vanguestaine, p. 194-196 (pro parte)non 1991 Frankea sartbernardensis var. A in André et al., p. 2901991 Frankea sartbernardensis var. B in André et al., p. 2901991 Frankea sartbernardensis in Servais, figs 17, 21, 23

Remarks:1. Servais (1993) provided measurements of 50 specimens

of F. sartbernardensis from Sart-Bernard, lower

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Llanvirn, Belgium. The process length against central body diameter ratio is about 1:5 to 1:6. This confirms Martin’s (1969) and Vecoli et al. (1999) ratio of 1/5. It is interesting to note that Martin (1969) observed that Ordovician specimens (comprising the holotype of Sart-Bernard) have a ratio process length against central body side length smaller than reworked Silurian specimens (pl. V, fig. 220, pl. VI, fig. 263 in Martin 1969). The latter specimen was transferred into F. breviuscula by Servais (1993).

2. Frankea sartbernardensis in Martin (1977) is clearly a Frankea hamata.

3. Frankea sartbernardensis var. A in André et al. (1991) are in fact F. breviuscula (see Vanguestaine, this volume).

4. The specimens observed have very short processes (ratio about 1/5) and generally fine striation on the central body ressembling the striation known in Arkonia tenuata.

5. Identification of specimens observed by Vanguestaine (1978) is confirmed for the Asquempont section. The specimens of the Quenast railway section (Quenast –3.386 and –3.650) could not be reinvestigated. Their identification is still to be confirmed. The species is however clearly recognized in the newly studied sample Quenast –3.770.

Occurrence: Unnamed member of the Asquempont canal and La Tourette sections; unit A (Martin and Rickards, 1979) of the Asquempont canal section; quartzophyllades de Quenast.

Genus Striatotheca Burmann, 1970Striatotheca quieta (Martin, 1969) Rauscher, 1974

Pl. II, Figs 11 and 12

1969 Veryhachium quietum n. sp. in Martin, p. 100-101, pl. V, fig. 226; pl. VI, fig. 290, text-fig. 481991 Striatotheca quieta (Martin, 1969) Rauscher in André et al., p. 2901991 Striatotheca quieta (Martin, 1969), Rauscher 1974 in Servais, pl. 1, fig. 81997 Striatotheca frequens Burmann, 1970 in Servais p. 62

Remarks:1. S. quieta with its short conical processes is easily

differentiated from S. frequens Burmann, 1970 which bears longer cylindrical, very gradually tapering processes with obtuse terminations.

2. Servais (1997, p. 62) mentioned few similar specimens to S. frequens found in the Rigenée Formation. He probably means S. quieta effectively found in that formation (Servais, 1991).

3. The holotype (Martin, 1969) is probably a reworked Ordovician specimen in Silurian strata. The drawing provided by Martin exhibits a densely striated membrane surface. The recorded specimens are on the contrary ornamented with widely spaced ribs.

4. First appearance of the species is unclear (Servais, 1997; Cooper et al., in press). In Belgium, the species is unknown in the Abbaye de Villers Formation and the Tribotte Formation but currently recorded in the Rigenée Formation of the Brabant Massif (Servais, 1991) and in the Huy Formation of the Bande de Sambre-et-Meuse (Martin, 1969, Servais & Maletz, 1992).

5. Two specimens with a similar striation on the central body have been observed of which one is reported as ?S. quieta due to the fact that the processes could be broken.

Occurrence: Unnamed member of the Asquempont canal section; Rigenée Formation in the Quenast quarry.

Striatotheca spp.

Remark:Due to the poor state of preservation, the distinction between some species as S. principalis parva and S. rarirrugulata are in general difficult to discriminate. This remark does not apply to the sample Virginal –8.343.

Genus Sylvanidium Loeblich, 1970?Sylvanidium operculatum Vavrdová, 1978

Pl. II, Fig. 13

1978 Sylvanidium operculatum n. sp. in Vavrdová, p. 68; pl. 11, fig. 4; pl. 12, figs 1-2; pl. 14, fig. 9; text-figs 2t-v1982 Sylvanidium operculatum in Vavrdová, p.149, pl. 3, figs 6, 71989 Sylvanidium operculatum in Vanguestaine et al., p. 451991 Sylvanidium operculatum in André et al., p. 240?1989 Acanthodiacrodium cf. simplex Combaz, 1968 in Rushton & Molyneux, p.272, fig. 4A

Remarks: 1. Sylvanidium operculatum is described by Vavrdová

(1978) in Llanvirn strata of Bohemia as a kind of Leiofusa with elliptical central body and relatively long processes, one at each pole, slighty offset away from the long axis of the vesicle. The presence of a median excystment aperture is an additional specific feature.

2. Rushton & Molyneux (1989) figured similar specimens cited as Acanthodiacrodium cf. simplex Combaz, 1968 in the upper Arenig of the English Lake District (Black Combe Inlier). No excystment structure is mentioned but dimension are similar to the ones of Sylvanidium operculatum except the process length.

3. Specimens, cited as ?Sylvanidium operculatum are mentioned in Vanguestaine et al. (1989) and André et al. (1991). They come from the lowermost part of the Rigenée Formation (sample Rigenée, chemin creux –2) in strata in which Dicrodiacrodium normale, Frankea sartbernardensis and Striatotheca quieta are

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 13

also observed (André et al., 1991, Servais, 1991 and Vanguestaine, unpublished). No excystment structure is visible.

4. The figured specimen (Pl. II, Fig 13) comes from La Tourette –7 sample. Dimensions of the figured specimens lies between the dimensions of Sylvanidium operculatum and Acanthodiacrodium aff. simplex. No excystment structure is clearly observed.

Occurrence: Unnamed member of the La Tourette section.

Genus Vogtlandia Burmann, 1970Vogtlandia multiradialis Burmann, 1970

Pl. II, Fig. 14

1970 Vogtlandia multiradialis n. sp. in Burmann, pl. IV, fig. 11979 Vogtlandia multiradialis Burmann, 1970 in Martin & Rickards, p. 191, fig. 2, p. 1931989 Vogtlandia multiradialis in Vanguestaine et al., p. 451991 Vogtlandia multiradialis in André et al., p. 2901991 Vogtlandia cf. multiradialis in Servais, p. 240-241, pl. 1, fig. 14

Remark:Burmann (1970) created 4 species assigned to a new genus Vogtlandia. They are distinguished on the base of process number and length. V. imperfecta and V. tenuata (3 processes), V. ramifacata (4-5 processes), V. multiradialis (7 processes). The generally poor preservation of the recorded specimens does not allow the distinction between the different species. Vogtlandia multiradialis is however clearly recorded in some samples, as Asq-11m.

Occurrence: Unnamed member of the Asquempont canal and La Tourette sections; questionable presence in the “quartzophyllades de Quenast”.

4. stratigraphic interpretation

Based on their acritarch content, the different studied samples are grouped into four distinct assemblages. The comparison with the Thyle section enable a stratigraphic interpretation.

4.1. Acanthodiacrodium angustum assemblage

The easily recognizable species Acanthodiacrodium angustum is identified in 9 samples from the Asquempont, Virginal and La Tourette sections. Acanthodiacrodium spp., Cymatiogalea spp. and Stelliferidium spp. are commonly found in this assemblage. Acanthodiacrodium ubuii Martin, 1969 is probably recorded (Pl. I, Figs 4-6) but left in open nomenclature due to the poor preservation. Moreover, the La Tourette section exhibits Vulcanisphaera flagellum specimens (Pl. I, Figs 11 and 12). This assemblage is similar to the assemblage found in the Chevlipont Formation at the Thyle river section (Martin,

1977, André et al., 1991). In Belgium, it has also been met in other places such as the Lessines borehole (Herbosch et al., 1991), the Wépion borehole (Martin, 1969) and the Jalhay Formation in the Stavelot Inlier (Vanguestaine, 1974, 1986), always in the same stratigraphical context. Index species of the late Tremadocian Stelliferidium trifidum – Cymatiogalea messaoudensis assemblage of the Lierneux Member (Stavelot Inlier, Breuer and Vanguestaine, 2004) are not encountered. Therefore, an early Tremadocian age is much probable. Full determination of Acanthodiacrodium ubuii would be interesting as the species is restricted to the lower part of the Tremadoc (Rasul and Downie, 1974).

4.2. Frankea hamata-Striatotheca rarirrugulata

assemblage

Sample Virginal –8.343 and, and in a lesser extent sample –8.347, yielded an assemblage very similar to the one observed in the middle and upper parts of the Abbaye de Villers Formation and the lower part of the Tribotte Formation of the Thyle river section. It is characterized by the co-occurrence of Frankea hamata, Frankea breviuscula and Striatotheca rarirrugulata. Adorfia firma and Arbusculidium filamentosum are also typical species of this assemblage. This assemblage is dated as late Arenig, (late Fennian, early Darriwilian) in the English Lake District (Cooper et al., in press). The lithostratigraphic attribution to the Abbaye de Villers Formation (Debacker et al., 2003) of the rocks yielding this assemblage in the Virginal section is in accordance with acritarch biostratigrahy though the Abbaye de Villers Formation and the lower part of the Tribotte Formation in the Thyle river section could not be discriminated based on their acritarch content.

4.3. Arbusculidium filamentosum-Frankea sartbernardensis assemblage

A particular assemblage, unknown so far in the Thyle river section, combines elements of assemblages recorded separately in that area. Adorfia firma and Arbusculidium filamentosum appear restricted to the middle and upper part of the Abbaye de Villers Formation and the lower part of the Tribotte Formation. While, Acanthodiacrodium tasselii, Arkonia virgata, Frankea sartbernardensis, Striatotheca quieta, ?Sylvanidium operculatum, Vogtlandia multiradialis are species recorded in the upper part of the Tribotte Formation and/or in the lower part of the Rigenée Formation.This particular assemblage is observed:(i) in the unnamed member of the Asquempont canal section (?Adorfia firma, Arbusculidium filamentosum together with Arkonia virgata, Frankea sartbernardensis, Striatotheca quieta and Vogtlandia multiradialis). (ii) in one sample at least of the Quenast section (sample 3.770) (Adorfia firma, Arbusculidum filamentosum together with Frankea sartbernardensis, Vogtlandia sp.).The specimens observed in the unnamed member at La Tourette (Frankea sartbernardensis, ?Sylvanidium

operculatum and Vogtlandia sp.), are supposed to belong to the same assemblage even if Adorfia firma and Arbusculidium filamentosum are not recorded.Based on acritarch evidence, this particular assemblage belongs to strata which could not correspond to the Abbaye de Villers, the lower part of the Tribotte and most part of the Rigenée formation. It has to be correlated to the middle part of the Tribotte Formation (where acritarchs are unknown) or with the upper part of the same Formation where Frankea sartbernardensis and ?Vogtlandia multiradialis are recorded. The turbiditic like facies of the unnamed member points rather to a lithostratigraphic position at the boundary between the Tribotte (shelf facies) and the Rigenée (deep environment close to the continental slope). The presence of Frankea sartbernardensis indicates an age not older than late Arenig (Cooper et al., in press; Vanguestaine, this volume).

Samuelsson & Verniers (2002) recorded chitinozoans in three of the localities discussed herein:(i) at Asquempont, in the unnamed member, where three samples (JV99-001, JV99-002, JV99-003 yielded an assemblage containing Desmochitina ornensis and Conochitina pseudocarinata indicating a mid Arenig age.(ii) at Quenast (sample JV99-066, km 3.777 of the railway), co-occurrence of Lagenochitina obeligis and Cyathochitina cf. dispar provides evidence for a late Arenig age, (iii) in the Thyle valley, the uppermost part of the Tribotte Formation contains Euconochitina vulgaris broadly indicative of a middle Arenig to early Llanvirn age.

If the three localities are more or less coeval, as suggested by acritarchs, combined acritarchs and chitinozoans results would indicate a late Arenig age.

4.4. Rigenée Formation acritarchs

Ten samples of the Rigenée Formation were tested in the studied area. The results are very thin:- Arkonia tenuata (Asquempont –39.7785, Quenast –2B)- Stelliferidium striatulum (Asquempont –39.7785)- Striatotheca quieta (Quenast –2B)- Striatotheca spp (Quenast –2B)- Veryhachium trispinosum (La Tourette –14, Asquempont –39.7785, –39.766, Quenast –2B, Queurcq –7)- Vogtlandia sp (Quenast –2B)

At Asquempont, in their unit D, Martin & Rickards (1979), recorded Frankea sartbernardensis and Striatotheca principalis.

Compared with the Rigenée Formation at Rigenée (Servais, 1991), acritarch diversity in the Senne-Sennette sections is very low. A typical species as Frankea hamulata known in the middle and upper part of the Rigenée Formation at Rigenée and Gembloux (Servais, 1991), is not observed in the Senne-Sennette.

This situation could be explained by a slight lateral facies variation rendering the depositional sites in the Senne-Sennette somewhat more reducing and/or in a more distal position in the basin lowering acritarch

specimen number. Moreover, it should be noted that the only place where graptolites are found in the Rigenée Formation is in the Sennette valley. At Rigenée, graptolites were never discovered and only trilobites are known (Servais, 1988, p. 21).

5. stratigraphical considerations

5.1. The Asquempont section

The Lower and Middle Ordovician rocks are discontinuously exposed at the southeastern trench of the new canal Bruxelles-Charleroi on a distance of about 350 m. Five lithological units are recognized from north to south, the Chevlipont Formation, an unnamed member (Verniers et al., 2002) and units A, B, C and D of Martin & Rickards (1979). The Acanthodiacrodium angustum and Arbusculidium filamentosum-Frankea sartbernardensis assemblages are recorded respectively in the Chevlipont and the unnamed member. Unit B of Martin & Rickards (1979) lithologically ressembles (Lenoir, 1987) the rocks of the Virginal railway section in which the Frankea hamata-Striatotheca rarirrugulata assemblage was observed but neither Martin & Rickards (1979) samples in this unit or Asquempont –39.974 (Fig. 2) sample revealed typical species of that assemblage. Unit A of Martin & Rickards (1979) yielded Frankea sartbernardensis in two samples. One specimen is figured (pl. I fig. 16). Vogtlandia multiradialis is also noted. Both species are unknown in the Abbaye de Villers and lower part of Tribotte Formation of the Thyle section but are found together (as ?Vogtlandia multiradialis) in the uppermost part of the Tribotte Formation (Vanguestaine, this volume) and in the Rigenée Formation (André et al., 1991; Servais, 1991). Therefore a correlation between Unit A of Martin & Rickards and parts of the Tribotte or the Rigenée Formations is possible. Unit C of Martin & Rickards (1979) presents specimens normally excluding each other in the Thyle section but found together however in the same places of the Senne-Sennette sections, notably in the unnamed member (Section 4.3.). They are Arbusculidium filamentosum (Abbaye de Villers Formation) on one hand and Acanthodiacrodium tasselii and Vogtlandia multiradialis (Rigenée Formation, see Vanguestaine, this volume; André et al., 1991; Servais, 1991) on the other hand. The record of such an assemblage could imply a correlation with strata of the Thyle section at the boundary between the Tribotte and the Rigenée Formations.

In conclusion, if the inferred stratigraphic comparisons with the Thyle section are correct, the succession at the south-eastern trench of the canal could be from north to south : the Chevlipont Formation, the unnamed member of Verniers et al. (2002) (possibly equivalent to the upper part of the Tribotte Formation), unit A of Martin & Rickards (1979) (possibly equivalent to the upper part of the Tribotte Formation), unit B (unknown stratigraphical equivalent), unit C (possibly equivalent to the upper part of the Tribotte Formation),

14� Michel Vanguestaine

unit D (= Rigenée). These correlations postulate that no diachronism exist between the Tribotte-Rigenée boundary in the Thyle section compared to the Tribotte (or Tribotte equivalent) –Rigenée in the Senne-Sennette valleys.

The succession, if correctly interpreted, is mainly composed of rocks younger than the Abbaye de Villers Formation except unit B of Martin & Rickards (1978).

Hennebert & Eggermont (2002) attributed the here described succession mainly to the Abbaye de Villers Formation and partly to the Tribotte Formation. Our results are in contradiction with their interpretation.

5.2. The Quenast railway section (Senne river valley)

According to Beugnies, in Waterlot et al. (1973), the succession along the railway trench, north of Quenast, between km 3.3 and 4.0, is composed from base to top of: a green slate formation, assigned to the Lower Tremadocian (= the Asquempont Member of the Oisquercq Formation), a transition zone of a few meters and the laminar siltstones of Quenast. This is the section studied in the present study. In fact, Beugnies mapped a southern unit, defined as composed of grey black shales and sandy slates and dated as Arenig, intercalated between the Quenast unit and the Rigenée Formation in all the Senne-Sennette valleys. This unit, at Quenast, was not studied for palynology. According to Herbosch (pers. comm.) it corresponds to the Tribotte Formation of the Thyle section. Outcrops of this unit exist at the northwest flank of the Senne valley in the “Bois du Chenoix”.

The transitional zone observed between km 3.378 and 3.386 and described as a green slate with centimetric layers of dark grey to black slate (Vanguestaine, 1978) has lithological features recalling the Lower Tremadocian Chevlipont Formation of the Thyle section. It was densely sampled by Vanguestaine (1978) and Lenoir (1987) (see Appendix 1). All samples are however barren or with useless stratigraphic specimens (Quenast –3.3785). This transitional zone could in fact correspond to a breccia linked to the Asquempont fault (sensu Debacker et al., 2003) separating the Cambrian Oisquercq Formation from the Ordovician rocks.

According to Hennebert & Eggermont (2002), the overlying “Quartzophyllades zonaires de Quenast” sensu Beugnies in Waterlot et al. (1973) would be the equivalent of the Abbaye de Villers Formation. Six samples in these laminar siltstones yielded microfossils. Five samples were already studied in Vanguestaine (1978). A sixth one (Quenast –3.770) is new. The acritarch composition of this last sample (Arbusculidium filamentosum-Frankea sartbernardensis assemblage) suggests a correlation with the upper part of the Tribotte or a level at the boundary between Tribotte and the Rigenée Formation (Section 4.3.). In any case, it is comparable with assemblages recorded in the Abbaye de Villers Formation of the Thyle section (Vanguestaine, this volume). Frankea sartbernardensis was identified in two other samples (km 3.368 and 3.650, Vanguestaine, 1978). The stratigraphic meaning of this species is important as it was never recorded below the uppermost part of the

Tribotte Formation. But despite new observations in the original slides, the identification of F. sartbernardensis could not be confirmed. These lithostratigraphic considerations therefore concern only one part of the unit called “Quartzophyllades zonaires de Quenast” in its type locality.

Based on chitinozoans, Samuelsson & Verniers (2000) dated as late Arenig a sample from the Quenast railway section (JV99-066, km 3.777). The lithological unit containing the sampled strata was considered as the Abbaye de Villers Formation. Acritarch results in a very nearby sample (km 3.770) agree with the late Arenig age, but does not support the lithostratigraphical attribution.

The lithostratigraphical confusion here described for the Senne valley confirmed other confusions already mentioned (Section 5.1.) in the Asquempont section where rocks mainly assigned to the Abbaye de Villers Formation (Hennebert & Eggermont, 2002) yielded acritarchs indicating rather the Tribotte Formation. The present considerations questioned the Middle Ordovician lithostratigraphy as presently applied in the Senne-Sennette river valleys.

5.3 Asquempont-La Tourette-Virginal section relationships

The three sections sampled in the Sennette valley are located relatively close to each others: the distance between the Asquempont and La Tourette section is 500 m ; between La Tourette and Virginal sections, about 580 m are measured along the Asquempont fault (sensu Debacker et al., 2003). Despite their proximity, the stratigraphical content of these sections is quite different. Above the Chevlipont Formation observed in the three sections, the unnamed member of Verniers et al. (2002) is found at Asquempont and La Tourette whereas the Abbaye de Villers Formation is observed at Virginal (Debacker et al. 2003).

On a biostratigraphical point of view, the Middle Ordovician unnamed member yielded at Asquempont the Arbusculidium filamentosum-Frankea sartbernardensis acritarch assemblage (Section 4.3.). A similar, but not identical, assemblage is found at La Tourette in the rocks reported to the same unit (Lenoir, 1987). This unit contains Frankea sartbernardensis. Absence of Arbusculidium filamentosum could be related either to the poor preservation or to some stratigraphic differences. According to the geological map of Debacker et al. (2003), the Asquempont and La Tourette sections belong to two different tectonic units separated by a normal fault called F7. Therefore, the two different sections do not necessary present coeval sequences of the unnamed member.

At Virginal, the Abbaye de Villers Formation is found succeeding the Chevlipont Formation. It yields the Frankea hamata-Striatotheca rarirrugulata Acritarch Zone (Section 4.2.). It was demonstrated earlier (Section 4.3.) that the latter zone, also found in the Abbaye de Villers and lower part of the Tribotte Formations in the Thyle section, is older than the Arbusculidium filamentosum-Frankea sartbernardensis assemblage. This

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 15

assemblage, though not recorded in the Thyle section so far, is probably partly observed in the uppermost part of the Tribotte Formation and most probably corresponds to strata near the boundary between the Tribotte and the Rigenée Formations.

According to the geological map of Debacker et al. (2003), the contact between the Chevlipont Formation and the non differentiated Middle Ordovician Abbaye de Villers and Tribotte Formations, between La Tourette and Virginal, is not underlined by a fault. The Middle Ordovician is represented as containing coeval strata, which acritarch biostratigraphy demonstrated to be at least partly wrong.

The discrepancy underlined above could be related to the fact that the Asquempont fault at La Tourette could affect not only Tremadocian strata and underlying rocks (see Debacker et al., 2003) but also overlying rocks, namely the Abbaye de Villers Formation. Other explanations could be found in unrecognized faults or in the outline of some recognized faults as the F10 fault between Virginal and La Tourette (see Debacker et al., 2003).

A faulted contact at La Tourette between the Chevlipont Formation and the unnamed member is probable as a Tremadocian specimen (Acanthodiacrodium angustum, see Section 3.1.3.) was found together with Middle Ordovician acritarchs. This situation is presumably related to the brecciated state of the studied strata.

6. conclusions

The Senne-Sennette river valleys offer the most complete succession of outcrops in the southern border of the Brabant Massif. However, due to their location in a highly tectonized zone and correlation difficulties with coeval strata in the Thyle river type section, Early and Middle Ordovician rock stratigraphy is relatively poorly known. The occurrence of a single graptolite horizon in one of the four described lithological units of the studied sequence rendered microfossil research even more important.

Lower Tremadocian assemblages are recognized in three places of the Sennette valley: the Virginal railway section, the Asquempont canal section and the La Tourette section along the right side of the old canal (Bief 29). All sections show the characteristic laminated facies of the Chevlipont Formation. Due to the fact that lower and/or upper contacts with adjacent strata are faulted, the exact thickness of the Formation in the Sennette area is unknown. Coeval strata in the Senne valley, Quenast railway section, are so far unknown.

A typical late Arenig acritarch assemblage belonging to the Frankea hamata – Striatotheca rarrirugulata Acritarch Zone of the English Lake District is newly identified in the Virginal railway section. It occurs in a lithological unit composed of decimetric grey-green sandstones in which plan parallel ichnofossils are developed. This unit was correctly assigned to the Abbaye de Villers Formation from the Thyle area in which the Frankea hamata – Striatotheca rarrirugulata Acritarch

Zone is also indentified (Vanguestaine, this volume).

A third acritarch assemblage is recognized in the unnamed member of the Abbaye de Villers Formation of Verniers et al., (2002) observable at Asquempont along the Bruxelles-Charleroi canal. This assemblage is characterized by the coexistence of ?Adorfia firma, Arbusculidium filamentosum on one hand with Acanthodiacrodium tasselii, Arkonia virgata, Frankea sartbernardensis, ?Striatotheca frequens and Vogtlandia multiradialis on the other hand, species currently occurring respectively either in the Abbaye de Villers Formation and the lower part of the Tribotte Formation or in the upper part of the Tribotte Formation and lower part of the Rigenée Formation in the Thyle section. A similar assemblage is also known from at least a part of the Quenast railway section which shows the coexistence of Adorfia firma, Arbusculidium filamentosum and Frankea sartbernardensis. Due to its stratigraphic position, it is also assumed that the La Tourette, Bief 29 section, with Frankea sartbernardensis, ?Sylvanidium operculatum and Vogtlandia cf. multiangularis also belongs to the same third assemblage.

Chitinozoan associations are recorded in two of the three localities cited above, the Asquempont and Quenast sections. Chitinozoans and acritarchs both indicate a stratigraphical position above the middle part of the Abbaye de Villers Formation. But, whilst chitinozoans would indicate a middle or late Arenig age, acritarchs would point to a late Arenig or early Llanvirn age, due to the presence of Frankea sartbernardensis and Arkonia virgata which known first appearance are respectively late Arenig and early Llanvirn in age. Combined acritarch and chitinozoan results probably indicate a late Arenig age.

On a lithostratigraphical point of view, the unnamed member of the Asquempont canal section and of La Tourette would better be placed as an equivalent of the middle or upper part of the Tribotte Formation or at the boundary between the Tribotte and the Rigenée Formations than near the Abbaye de Villers – Tribotte boundary as previously suggested. Part at least of the “Quartzophyllades zonaires de Quenast” at Quenast occupies the same position. It is also suggested that most of the rocks occurring at Asquempont between the Chevlipont and the Rigenée Formations are coeval with the Tribotte Formation of the Thyle valley.

While in the type section of the Rigenée Formation, the Thyle river area, relatively good acritarch assemblages were observed, very few specimens are identified in coeval strata of the Senne-Sennette area, despite the number of samples tested.

The present paper emphasizes the importance of acritarch biostratigraphy to unravel the order of strata involved in strongly faulted tectonic structures. The succession in the time of the first apparition of the Frankea species hamata, breviuscula and sartbernardensis was particularly helpful. The described assemblages do not take in account all present species but only selected ones, with no incidence on our biostratigraphic conclusions. A better biostratigraphy could be obtained after a complete

16� Michel Vanguestaine

description of all Lower and Middle assemblages is achieved.

appendix: sample location

1. Asquempont canal section

The quartz veins (km 40.118 – 40.119 according Debacker et al., 2003) associated to the Asquempont fault and/or the distance with respect to km 40 along the canal have been used to measure distances. At the time of sampling, the quartz veins were situated at km 40.133. The Km –post 40 in the meantime, has been moved and the distances given herein do not match that of the newly places km –post 40.

Chevlipont Formation

Asq –3, –5 (in Lenoir, 1987): 3 m and 5 m. south of the quartz veins.

Unnamed member of the Abbaye de Villers Formation

Asq –5 (sic), –28 (in Vanguestaine, 1978) and Asq –5,5, –11 in Lenoir (1987).

Unit A of Martin & Rickards (1979)

Asq –77 (= km 40.056) more or less corresponding to FM –76 –5.3.

Unit B of Martin & Rickards (1979)

Asq –39.974 (also called Asquempont 3): at the northern corner of the concrete plate n°15

Rigenée Formation (= Unit D of Martin & Rickards, 1979)

Asq –39.766 (also called Asquempont 2, northern corner of the outcrop) and –39.7785. (Three other samples taken in the same formation are barren).

2. Virginal railway section

The samples are located according to the railway kilometers, all from the western side of the trench.

Chevlipont Formation

Virginal –8.1883, –8.1868, –8.1884 and –8.190 are four samples taken near the faulted contact with the Oisquercq Formation. They are taken from different height and therefore represent no more than about 2 meters of strata (for more details see Lenoir, 1987).Virginal –8.200, –8.216 and –8.2224 are taken further to the south presumably in a higher stratigraphic position in the Formation.

Abbaye de Villers FormationTwo samples, at km 8.343 and 8.347, in a lithological unit composed of decimetric grey-green sandstones in which bedding plane parallel ichnofossils are developed. The lithostratigraphic attribution is from Debacker et al. (2003)

Rigenée Formation

Virginal –8.552.

Remark : The acritarch content of samples –8.200, –8.216 and –8.347 is also provided in Vanguestaine (1978). Sample 8.343 is a newly studied sample.

3. La Tourette section

Eleven samples are partly localized with respect to a stair, taken as point 0, in the garden of the property n°11. They are herein numbered according Lenoir (1987) but often named differently in the collections of Liège University (see synonymy).

Chevlipont Formation (in m. west of point 0)La Tourette –1 (= La Tourette –6) and –2 (=P1): 13 m.La Tourette –3 (= P2): 16,5 m.La Tourette –4 (= P3): 25 m.La Tourette –5 (= La Tourette –7) and –6 (= F1) at 29 m and 4 m east of a close.

Unnamed member of the Abbaye de Villers FormationLa Tourette –7 (= F2), –8 (= F3), –9 (= F4), –10 (= F5) and –11 (= P4) in a sequence comprised between 29 and 30,5 m from point 0. Sample La Tourette –11 is at 2,5 m east of the close.La Tourette –12 (= La Tourette –5) and –13 in the outcrop bearing the galena dyke west of the close of the property n°11.

Rigenée FormationLa Tourette –14, about 15 m west of previous outcrops, from a sequence of homogeneous black slates.

4. Coeurq river section

Middle Ordovician laminar siltstones (no lithostratigraphical attribution)

Coeurq –1 in the northernmost outcrop, near a little concrete bridge.Coeurq –2 in the Coeurq stream: black slate.Coeurq –3, –5, –6: near the Coeurq fall.Coeurq –4: along a road near the “Moulin du Coeurq”.

Rigenée FormationCoeurq –7 : in an outcrop at the “Ferme des Aunois”, at 600 m northeast of the Hennuyères station.

5. Quenast railway section and quarry

Transitional zone (Vanguestaine, 1978)

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 17

Six samples are localized according the railway kilometers. From north to south: Quenast –3.3767, –3.378, –3.3785, –3.3794, –3.3805 and –3.3829. Underlined samples are from Vanguestaine (1978). The others were collected by Lenoir (1987).

“Quartzophyllades zonaires de Quenast” according Beugnies in Waterlot et al. (1973)

Five samples in Vanguestaine (1978): Quenast –3.368, –3.415, –3.436, –3.517, –3.650. A sixth sample newly studied herein is located at km –3.770, opposite Bloc B2. A typographic error in Vanguestaine (1978) has alterated the localization of the sample Quenast –7 which has to be red –3.750 instead of –3.650. The newly studied sample –3.750 is a topotype sample of Quenast –7 (Vanguestaine, 1978).

Rigenée Formation

Three samples, Quenast –1a, Quenast –2a, Quenast –2b are respectively taken in the Quenast quarry at the contact with the intrusive rocks (–1a) and at two places 10 m off the contact where a fault separates the shale from the intrusive rock itself (–2a, –2b).

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18� Michel Vanguestaine

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Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 19

VAVRDOVÁ, 1978. Nethromorphitae and some other acritarchs from the Bohemian Lower Ordovician. In : V. Pokorny (ed.), Paleontologickà Konference Katerdy Paleontologie na Prirodovedecké Fakulte Univerzity Karlovy, Praha, p. 61-74, pl. 11-14.VAVRDOVÁ, 1982. Phytoplankton communities of Cambrian and Ordovician age of central Bohemia. Vestnik Ustredniho Ustavu Geologickeho, 57 : 3 : 145-155, pl. 1-4.VECOLI, M., TONGIORGI, M. & PLAYFORD, G., 1999. The Ordovician acritarchs Frankea breviuscula, F. longiuscula, and F. sartbernardensis: a new study. Bollettino della Società Paleontologica Italiana, 38: 343-358.VERNIERS, J., HERBOSCH, A., VANGUESTAINE, M., GEUKENS, F., DELCAMBRE, B., PINGOT, J.-L., BELANGER, I., HENNEBERT, M., DEBACKER, T., SINTUBIN, M. & DE VOS, W., 2002. Cambrian-Ordovician-Silurian lithostratigraphic units (Belgium). In: Bultynck, P., Dejonghe, L. (Eds), Guide to a Revised Stratigraphic Scale of Belgium, Geologica Belgica, (4): 5-38.WATERLOT, G., BEUGNIES, A., BONTE, A., CHARLET, J.-M., CORSIN, P., BINTZ, J., HARY, A. & MULLER, A., 1973. Ardenne-Luxembourg. Ardennes. Guides Géologiques Régionaux. Itinéraire 10 : 89-98. Masson, Paris.

Manuscript received on 01.04.2005; accepted on 15.11.2005; published on line on 01.08.2007

plate i

(Acritarch specimens from the lower Tremadocian Chevlipont Formation (1-12) and the Middle Ordovician unnamed member (13-15) in the Sennette valley sections)

(All microphotographs at 1000x)

Fig. 1 Acanthodiacrodium achrasii Martin, 1972Virginal –8.222, 21499, J53/4

Figs 2 and 3 Acanthodiacrodium angustum (Downie, 1958) Combaz, 1967

2: La Tourette –7, 21629, S38/13: La Tourette –6, 21635, H35/1

Figs 4, 5 and 6 Acanthodiacrodium ubuii Martin, 1969

4: Virginal –8.222, 21499, N-0395: Asquempont –3, 21502, Q46-476: Virginal –8.222, 21499, R51/3

Fig. 7 Cymatiogalea sp.

Virginal – 8.222, 21499, M54 Figs 8, 9 and 10 Stelliferidium spp.

8: La Tourette –6, 21628, U47/19: Virginal –8.222, 21499, F5310: Virginal –8.200, 6112, JK44

Figs 11 and 12 Vulcanisphaera flagellum Martin, 1977

11: La Tourette –2, 21811, N46/412: La Tourette –6, 21628, K52/2

Figs 13 and 14 Acanthodiacrodium tasselii Martin, 1969

13: Asquempont –11 m, 21687, U42/4 : one of the poles exhibits well preserved processes.14: La Tourette –7 m, 21629, S49/2 : damaged specimen with broken processes.

Figs 15 ?Adorfia firma Burmann, 1970

Asquempont –7 m, 21687, K44-K45: with unclear distal bulbous termination.

20� Michel Vanguestaine

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 21

plate ii

(Acritarch specimens from Middle Ordovician strata of the Senne-Sennette river sections)

(All microphotographs at 1000x)

Fig. 1 ?Adorfia firma Burmann, 1970Asquempont –7 m, 21687, L55-56: unclear pedonculate balls in the ultimate pinnulae

Fig. 2 Arbusculidium filamentosum (Vavrdová, 1965) Vavrdová, 1972

Asquempont –7 m, 21676, P38/4 : distinct filamentous broken processes at one pole. Fig. 3 Arkonia virgata Burmann, 1970

Asquempont –7 m, 21685, M34/3 Fig. 4 Arkonia sp. cf. Arkonia virgata (Burmann, 1970)

Asquempont –77m, 4586, P36/2 Fig. 5 and 6 Aureotesta clathrata simplex (Cramer, Kanes, Diez and Christopher, 1974)

Brocke, Fatka and Servais, 19778: Asquempont –7 m, 21684, R48/39: Asquempont –7 m, 21676, D42/1

Fig. 7 Coryphidium sp. cf. Coryphidium bohemicum Vavrdová, 1972

Asquempont –77 m, 4586, J38/1 Fig. 8 Dasydorus sp. cf. Dasydorus cirritus Playford and Martin, 1984

La Tourette –7, 21629, G56/3-4: one of the poles is distinctly smooth. Figs 9 and 10 Frankea sartbernardensis (Martin, 1966) Colbath, 1986

12: La Tourette –7 m, 21636, D48/113: Asquempont –7 m, 21685, T44/4

Fig 11 Striatotheca quieta (Martin, 1969) Rauscher, 1974

Quenast –2b, 21411, M49/3 Fig. 12 ?Striatotheca quieta (Martin, 1969) Rauscher, 1974

Asquempont –7 m, 21684, O42 : possibly broken processes. Fig. 13 ?Sylvanidium operculatum Vavrdová, 1978

La Tourette –7 m, 21629, V44/4 : short (? broken) processes; ?lateral excystment structure. Fig. 14 Vogtlandia multiradialis Burmann, 1970

Asquempont – 11 m, 21687, LM42: stellate specimen bearing at least 7 processes.

22� Michel Vanguestaine

Early and Middle Ordovician acritarchs of the Senne-Sennette river valleys� 23